Tuesday, July 10, 2012

Higgs Boson Expected End in Particle Physics

Mark Wyman

The Fourth of July announcement that the long-sought Higgs Boson had been found has led physicists – professional and armchair -- around the world to celebrate. The Higgs, often (bizarrely and unhelpfully) referred to as the ‘God particle’, was the last missing particle in the ‘standard model’ of particle physics, and its discovery at CERN’s Large Hadron Collider (LHC) is in a way the last great triumph of 20th century science.

Despite the celebrations, the question that haunts the physics community is whether the LHC will now begin to start finding new things, opening the door to another century of discovery, or if the Higgs finding will represent the final chapter of discovery in particle physics.

The Higgs itself is a strange beast. Every other particle we know exists falls into two categories: fermionic ‘matter’ fields, like electrons and quarks, that largely make up the stuff of the Earth, and bosonic ‘force carriers’, like photons, that are the delivery agents of the fundamental forces. The Higgs is different: it mediates no force itself, but instead is the means by which the ‘matter’ fields gets their masses.

To understand how this happens, we must begin by remembering that all ‘particles’ are associated with ‘fields’ that fill all space. The electron and, say, quark fields thus actually have a lot more in common with the ‘Force’ of Star Wars imagination than they do with the billiards balls we often picture.

The eccentric (and brilliant) physicist John Ellis proposed an elegant analogy for how the Higgs field works (see video here). Imagine the Higgs field as freshly fallen snow. Other particles have to travel across this field to get where they’re going. Some particles – the ‘heavy’ ones, like the exotic top quark or tau lepton – sink heavily into the snow with each step, proceeding slowly. Others, like the very light electron proceed more easily, as if wearing skis. And still others – like the photon, the particle name for light itself – are like Legolas, the elf in The Lord of the Rings, and proceed as if there is no snow there at all.

For theoretical particle physicists, the important thing about having a Higgs to give particles their mass is that it allows the fundamental laws of physics to have no masses whatsoever. The fact that the mathematics that describe these laws imply that there are no fundamental masses is what spurred Peter Higgs and others to propose the ‘Higgs mechanism’ in the first place in the early 1960s.

What does this all mean for physics? In the short term, nothing changes: the Higgs was predicted nearly 50 years ago, and its existence has been assumed, if not proven, for years. That some physicists – most famously Stephen Hawking – had hoped (and even made bets) that the Higgs wasn’t there was, in my view, more likely a hopeful wish that the Universe had more surprises for us than a considered calculation that the predictions had been wrong.

That the predictions were (largely) correct is good news in the sense that we can be confident that we know how to do calculations, but it may be bad news for everyone seduced by the adventure of discovery. This is because, after the Higgs, there are no more particles that are definitively predicted by already-existing data (hints of a Higgs-like particle have been seen in other physics experiments for more than a decade).

Physicists have many arguments that the Higgs can’t be the end of the line, though. At a minimum, we are sure that the dark matter that comprises most of the matter in the Universe is not part of the ‘standard model’. What we don’t know is if Earth-based particle detectors will ever be able to find the other new particles we suspect might be there.

On the other hand, there are already clues that the Higgs we have found isn’t a boring one: there are subtle mismatches between the data we have so far about the Higgs that was seen by the LHC and the Higgs that was hinted at by previous experiments. Whether that’s due to mistakes or peculiarities of small calculational details or is the first thread in the unraveling of the standard model is something that we all hope the LHC will be able to tell us.

I previously called the Higgs the final great discovery of 20th Century science quite deliberately. The scientists who predicted its existence are now old men, and it’s the fondest hope of those of us who are still young in the field that there are still unexplored veins of accessible physics for us to discover.

Finally, we can ask what this milestone means for the wider world. Without any doubt, it is an important step forward in human knowledge. There are few things more fundamental to know about our Universe than its most basic building blocks, and the Higgs –whether the standard model is replaced by new theories or not – is one of those building blocks.

People sometimes mutter about the high cost of modern science budgets (the LHC has cost around US$10 billion so far) and ask if it’s worth it for society to spend so much money on something with no practical benefits (the Higgs is unlikely to help in the quest to cure cancer or to make your computer run faster).

I like to remind them that in North America alone about $10 billion is spent per year on movie tickets, which are also not notable for their contribution to cancer research or any other practical ends. The quest to understand the basic facts of the Universe is one of the highest pursuits of the human mind, and the need to know is an irreducible aspiration of the human spirit. The privilege of pursuing science as a vocation is one that only is available to lucky individuals in wealthy societies, but I believe that we all know that the construction and testing of scientific theories is akin to the building of great cathedrals or the effusion of Catherine wheels and Roman candles on the Fourth of July: the purpose and culmination of civilization and the height of earthly human ambition.

Mark Wyman is a Research Associate in the Department of Astronomy and Astrophysics at the University of Chicago.